Pressure vessels are used in many areas of plant engineering, mechanical engineering and automotive construction in numerous applications. As a rule, they are structured either spherically or else have an elongated cylindrical shape, whereby the end sections are hemispherical or semi-ellipsoidal or else are configured as a torispherical head. Typical areas of use for such pressure vessels include tanks for compressed gases, for example, natural gas (CNG) or hydrogen (H2), or hydraulic accumulators, for instance, for the emergency lubrication of systems or for brake energy regeneration in trucks.
Pressure vessels were originally configured as load-bearing solid metal tanks made of steel, aluminum or other metal materials. However, especially in conjunction with mobile applications, possibilities for weight reduction were sought. Today's state of the art comprises so-called type IV pressure vessels for which a liner that is resistant and tight vis-à-vis the filling and/or operating media and that is essentially not load-bearing and made of plastic is surrounded on the outside by a load-bearing outer shell made of filaments embedded in synthetic resin.
The classification of the vessels can be found in ISO standards 11119-1 to 11119-3 and as such, has also been incorporated into ECE regulation 110. In short, it can be said that the Type I vessel is made of metal, the Type II vessel has a metal liner that is reinforced with a continuous filament impregnated in resin that is only wound in the cylindrical area in the direction of the circumference, while the end sections are not reinforced, the Type III vessel has a metal liner and is reinforced with a continuous filament impregnated in resin that is completely wound around the metal vessel, and the Type IV vessel has a non-metal liner (full composite construction) that is likewise completely reinforced with a continuous filament winding embedded in a resin matrix.
The liner used for the Type IV vessel is also referred to as a core tank. As a rule, this is a body that is made of suitable plastics such as, for example, polyethylene (PE), polypropylene (PP) or polyamides (for example, PA6, PA12) by means of a blow-molding or rotational-molding method, whereby the connection elements needed for a given application are integrated into at least one of the two end sections.
According to the state of the art, which is described, for example, in U.S. Appln. No. 2007/0164561, in order to produce the load-bearing outer shell, the liner is wound with continuous filaments or with rovings that are impregnated with liquid, curable thermosetting resins. Such resins are especially epoxy, phenol, polyester or vinyl ester resins. After the wet-winding process, the resin matrix is cured, as a rule, at a high temperature.
As an alternative, filaments or rovings that have been pretreated or impregnated with thermoplastics are used in the winding process. Methods for their production are described, for example, in U.S. Appln. No. 2002/0150752. For the subsequent solidification of the plastic matrix in which the filament winding is embedded, a sintering process at a high temperature is required after the winding, and in this process, the thermoplastic matrix that embeds the filaments is solidified. Such a process is known, for example, from U.S. Pat. No. 6,605,171 B1.
European patent document EP 0 333 013 describes a special method for winding the liner with the filament material.
German patent documents DE 197 51 411, DE 100 00 705 and DE 10 2006 004 121 as well as European patent document EP 1 989 477 do not deal with the production of the pressure vessel as such but rather, they describe various methods as to how a connecting section, neck piece or the like can be shaped onto or integrated into the liner.
Irrespective of whether a thermosetting matrix or a thermoplastic matrix is used to embed the filament winding in the outer shell and how they are further processed, however, such pressure vessels of Type IV entail a number of drawbacks.
When pressure vessels are produced with a thermosetting matrix in the fiber-reinforced outer shell, the wet-winding process using impregnated filaments calls for frequent downtimes so that the installations and the surroundings can be cleaned from the splashed resin material that sprays or drips off the impregnated filaments during the further processing. Subsequently, the vessels are placed for several hours in ovens or tunnel furnaces at a high temperature in order to cure the thermosetting resin material. At times, this has to be done while continuously rotating the workpieces so that the initially still liquid resin does not drip off. All in all, a greater investment in terms of time and costs is involved, which has a detrimental effect on the cost-efficiency of such a method. Moreover, the presence of the thermosetting matrix means that the pressure vessel cannot be recycled through the modality of material recovery at the end of its service life.
In contrast to reinforcement with a thermosetting matrix, pressure vessels having filament-plastic composite reinforcement with a thermoplastic matrix can be recycled through the modality of material recovery, especially if the liner and the matrix are made of the same thermoplastic. It is a drawback, however, that, as described in U.S. Appln. No. 2002/0150752, the process that precedes the wrapping, namely, the production of the filaments provided with the thermoplastic, is complex and constitutes a major time and cost factor.
However, if the filament is impregnated with polymer thermoplastic matrix material immediately before the liner is wound, so that the filaments are wound onto the liner even before the plastic cools off and solidifies, then, as is the case with the wet-winding process using thermosetting plastics, the surroundings will get dirty from dripping and splashing.
In principle, with the use of a thermoplastic matrix, it would also be conceivable to carry out the winding with a filament that is not provided with matrix material and to subsequently incorporate a thermoplastic matrix in which the filaments would then be embedded. However, it has been found here that a complete impregnation—that is to say, without voids or defects—of the wound filament packet is practically not possible, so that the resultant impregnation of the filament material with the matrix material is inadequate. As a result, no pressure vessel of Type IV can be produced that reliably meets the quality assurance requirements.
Therefore, it is an objective of the invention to provide a method in which a pressure vessel of Type IV can be produced without the need for frequent cleaning of the system or for long throughput times in the process chain, and which nevertheless reliably impregnates the filaments.